It is to be appreciated that any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the present invention. Further, the discussion throughout this specification comes about due to the realisation of the inventor and/or the identification of certain related art problems by the inventor. Moreover, any discussion of material such as documents, devices, acts or knowledge in this specification is included to explain the context of the invention in terms of the inventor's knowledge and experience and, accordingly, any such discussion should not be taken as an admission that any of the material forms part of the prior art base or the common general knowledge in the relevant art in Australia, or elsewhere, on or before the priority date of the disclosure and claims herein.
The overall reaction of water splitting, 2H2O→2H2+O2, produces O2 and H2 gases as end products. These gases need to be kept separate for later individual use and to avoid production of an explosive gas mixture (Tributsch H. Photovoltaic hydrogen generation Int J Hydrogen Energy 2008; 33:5911-30). There are several approaches to the design of devices that can maintain separation of the two gases during electrolysis, for example the use of a membrane to separate the electrode compartments. This also minimizes cross-over of dissolved gases from one electrode to be recycled at other electrode (Ioroi T, Oku T, Yasuda K, Kumagai N, Miyazaki Y. Influence of PTFE coating on gas diffusion backing for unitized regenerative polymer electrolyte fuel cells. J Power Sources 2003; 124:385-9; and Marangio F, Pagani M, Sentarelli M, Cali M, Concept of a high pressure PEM electrolyser prototype. Int J Hydrogen Energy 2011; 36:7807-15.
Although the gases can be separated, new issues arise with these technologies e.g. cost, mechanical properties, high resistance through the membrane and ultra pure water is needed for proper operation (Nieminen J, Dincer I, Naterer G. Comparative performance analysis of PEM and solid oxide steam electrolysers. Int J Hydrogen Energy 2010; 35:10842-50). Alkaline zero gap electrolysers using OH− conducting membranes are also being considered (Pletcher D, Li X. Prospects for alkaline zero gap water electrolysers for hydrogen production. Int J Hydrogen Energy 2011; 36:15089-104).
In the traditional alkaline electrolyser, where a diaphragm is the only separator, bubble formation inside and between the electrode and the separator is the major cause of transport resistance. A number of suggestions on bubble management have been made e.g. use of mechanical circulation of the electrolyte, use of (stable) additives to reduce surface tension of the electrolyte so the bubble can leave the system easier and modification of the electrode surface properties to be less attractive to the gas bubbles (Zeng K, Zhang D. Recent progress in alkaline water electrolysis for hydrogen production and applications. Prog Energy Combust Sci 2010; 36:307-26.)
One of the features of the O2 evolution reaction is that the dissolved oxygen concentration at the electrode has to build up to a level sufficient to nucleate and form small, high-pressure bubbles. According to Laplace's equation: P=2γ/r, where P is pressure in the bubble, γ is the surface tension and r the radius of the bubble, near the surface of an electrolyte, O2 bubbles with 0.1 μm radius need to have a pressure of 14 atm at 25° C. The concentrations required not only produce overpotential at the electrode (and thus inefficiencies in water splitting), but also represent a very reactive environment that challenges the long term stability of many catalysts.
Several reports have described efforts to improve water splitting cell efficiency by addition of sacrificial agents or co-catalysts, modification of catalyst crystal structures and morphology, and specific surface area (Kudo A, Miseki Y. Heterogeneous photocatalyst materials for water splitting. Chem Soc. Rev 2009; 38:253-78; Kato H, Asakura K, Kudo A. Highly efficient water splitting into H2 and O2 over lanthanum-doped NaTaO3 photocatalysts with high crystallinity and surface nanostructure. J Am Chem Soc 2003; 125:3082-9; and Osterloh F E. Inorganic materials as catalysts for photochemical splitting of water. Chem Mater 2008; 20:35-54.)
A few reports have designed novel electrode architectures at the nano- or micro-scopic scales to enhance cell performance (Mohapatra S K, Misra M, Mahajan V K, Raja K S. Design of a highly efficient photoelectrolytic cell for hydrogen generation by water splitting: Application of TiO2-xCx nanotubes as a photoanode and Pt/TiO2, nanotubes as a cathode. J Phys Chem C 2007; 111:8677-85; and Yin Y, Jin Z, Hou F. Enhanced solar water-splitting efficiency using core/sheath heterostructure CdS/TiO2 nanotube arrays. Nanotechnology 2007; 18).
Also, there have been attempts to separate the gases using different flow streams of the electrolyte in a planar microfabricated device, but the device efficiency was not high (Jiang L, Myer B, Tellefsen K, Pau S. A planar microfabricated electrolyzer for hydrogen and oxygen generation. J Power Sources 2009; 188:256-60). It appears that improvements, based on modification of the electrode structure, to rapidly remove the O2 from the cell before the bubble is formed, has not yet been widely considered. The traditional gas diffusion electrodes (GDE) of the type used in fuel cells have a tendency to continue to form O2 bubbles when operating in water splitting devices (Ioroi T, Oku T, Yasuda K, Kumagai N, Miyazaki Y. Influence of PTFE coating on gas diffusion backing for unitized regenerative polymer electrolyte fuel cells. J Power Sources 2003; 124:385-9). Moreover, these electrodes are not stable under water oxidation (WO) condition, the carbon being rapidly oxidized at the potentials involved in WO (Chaparro A M, Mueller N, Atienza C, Daza L. Study of electrochemical instabilities of PEMFC electrodes in aqueous solution by means of membrane inlet mass spectrometry. J Electroanal Chem 2006; 591:69-73; and Jang S E, Kim H. Effect of water electrolysis catalysts on carbon corrosion in polymer electrolyte membrane fuel cells. J Am Chem Soc 2010; 132:14700-1.)
In other prior art work a hydrophobic gas porous membrane (Goretex®) has been used to develop an efficient three phase-interface structure for an air-electrode (Winther-Jensen B, Winther-Jensen O, Forsyth M, MacFarlane D R. High rates of oxygen reduction over a vapour phase-polymerized PEDOT electrode. Science 2008; 321:671-4). The advantage of this, as a substrate for an electrode, is that gas can diffuse through the membrane, but liquid water cannot, and that an efficient three-phase interface can be maintained during operation. The fact that the cell responds linearly to the O2 content in the supplied gas in the O2 reduction reaction clearly proved that efficient gas transportation through the electrode was achieved.